Fundamental Concepts of Child Development
HOMEOSTASIS AND ADAPTATION
W. Thomas Boyce and Jack P. Shonkoff
No single construct has been more central in the development of the biologic sciences than homeostasis. Although the term homeostasis was coined in the 20th century, its conceptual origin can be traced to the notion of a stable, relatively unchanging internal environment, which was first described by Claude Bernard in the 19th century. Bernard recognized the fragility of life, surrounded as it is by a constantly threatening, aversive, and often pathogenic environment, and he argued that viability in the face of external challenge depends on an organism’s capacity for protecting its internal milieu. In this context, homeostasis is a dynamic, self-regulating process that ensures constancy and permanence in the internal physiologic state through complex, multilevel feedback systems that respond to a deviation in one direction with a countering adjustment in the opposite direction. Thus, the fundamental goal of a homeostatic system is to maintain an inerrant “set point” that assures stable and continuous biologic functioning. The regulation of body temperature, cortisol suppression of adrenocorticotropic hormone (ACTH) secretion, and glycogenolysis during periods of hypoglycemia are all examples of feedback loops that protect the continuity and equilibrium of an organism’s interior.
Whereas homeostasis governs regulatory strategies within the tissue, cell, or subcellular structures, the closely related concept of adaptation refers to the behavioral and biologic activities that promote the survival of individuals or groups. In evolution, adaptation involves the selective preservation and reproduction of organisms and characteristics of organisms that offer survival benefits in the face of external threats.1 Beyond evolution, however, adaptation has been used to describe complex social and individual developmental processes that respond to specific environmental challenges. For example, daytime continence emerges in a 3-year-old child within a context of growing parental expectations for toilet training; a preschooler clings to a tattered but revered blanket (a so-called transitional object) to calm his or her uncertainties and fears about attending a new childcare center; and a 12-year-old girl exhaustively discusses her first menstrual period with friends as a means to cope with the complications and challenges of sexual maturation. At all stages of development, the capacity to weather, absorb, and find meaning in the vicissitudes of life is one of the defining characteristics of humankind.
Many novel, challenging life experiences are commonplace and are accessible to a range of homeostatic and adaptive strategies. However, children also encounter circumstances that strain their adaptive capacities and may present acute or chronic stressors that exceed their ability to cope. Indeed, psychosocial stress has been defined as those environmental demands or threats that overtax an individual’s ability to adapt. When such conditions are encountered, a variety of biologic and behavioral responses are evoked; if sufficiently intense or prolonged, such responses can lead to the development of a diagnosable disorder.
Research in humans suggests that 2 principal, interrelated systems are involved in the neurobiologic response to stress: (1) the corticotropin-releasing hormone system, and (2) the locus ceruleus–norepinephrine system.2
These two highly interactive response systems mediate successful neurobiologic adaptation to stressors but play a contributing etiologic role, as well, in the pathogenesis of stress-related physical and mental health disorders. In some cases, such disorders constitute a failure of homeostatic and adaptive processes; in other cases, they reflect the capacity of dysfunctional, exaggerated adaptive processes to cause disease. Associations between emotionally stressful experiences and adverse, maladaptive health outcomes have become increasingly well substantiated in both adults and children. Although debate continues about whether stressors cause specific pathologic conditions or simply alter generalized host susceptibility, little doubt remains that both chronic adversities and acute stressful events elevate risks of physical and mental disorders.
Both clinical experience and epidemiologic observations suggest that not all children are equally vulnerable to psychosocial stressors. In fact, homeostatic and adaptive capacities appear to be quite un evenly distributed within human populations.3 Some children succumb to a succession of modestly stressful events, while others seem able to sustain normal functioning and health through even the most adverse and emotionally trying circumstances. Observations of children with varying levels of neurobiologic sensitivity to environmental challenges raise questions about the universality of stress-illness linkages and underscore the importance of individual differences in children’s behavioral and biologic responses to the social world.
While observations regarding such differences extend back to ancient Greek civilization, the systematic study of temperamental differences began with the New York Longitudinal Study of Stella Chess and Alexander Thomas. Although researchers differ in the extent to which they view temperament as a stable, inherent characteristic, all agree that the concept itself describes a set of individual predispositions that underlie and modulate the expression of activity, emotionality, and sociability. In this study, clusters of behavioral styles were constructed to identify “easy,” “difficult,” and “slow-to-warm-up” children.4 Further study suggests that temperament has both behavioral and neurobiologic aspects. Behavioral differences generally are arrayed along dimensions such as activity level, adaptability, intensity, and mood; neurobiologic differences include the physiologic responses to stressors and challenges that reflect internal reactivity to environmental events.
The origins of differences in behavioral and psychobiologic “style” are not completely known, but they appear to be interactively determined by genetically based predispositions, environmental exposures, and the epigenetic effects of experience on gene expression. For example, individual differences in shyness and timidity may be plausibly derived from constitutional differences in gene sequences, from personal experiences, and/or from experiential modifications of the epigenome (that is, the packaging or structural state of DNA) that are capable of changing gene transcription.5
An appreciation of individual temperament differences is important in the practice of pediatrics not only because of their impact on development and behavior but also because of their potential link to both mental and physical health. For example, preschool children with extreme shyness may be at heightened risk for anxiety disorders during middle childhood or for panic disorder and agoraphobia (ie, fear of being in large, open spaces) as adults. Children with exaggerated cardiovascular or immunologic responses to stressors appear to have elevated incidences of injuries and respiratory infections during periods of naturally occurring stressors such as residential moves or parental divorce.
While the mechanisms underlying these associations are debated, available data suggest that certain subsets of children may have an impaired ability to self-regulate their behavior, physiologic functions, and subjective experiences of somatic pain. One possible explanation for impaired self-regulation is that children who display certain behavioral and neurobiologic phenotypes (eg, shyness and its associated predisposition to autonomic arousal) have an underlying, heightened sensitivity to the social environment and a relative inability to monitor and constrain their behavioral and physiologic responses.
A capacity for recognizing, monitoring, and containing the emotional “coloring” of environmental events is a critical early developmental achievement. Infants’ interactions with caregivers, principally with their mothers, guide and shape the unfolding of affective experience and expression during the first months of life. Later, as maturation proceeds, the regulation of emotional experience becomes less dependent on caregivers and more accessible to a child’s emerging self-control. Such individual differences in the capacity for self-regulation may emerge as an important determinant of mental and physical well-being. In a moment of prescience 250 years ago, Thomas Sydenham wrote that the cause of “nervous disorders” may lie partially in “the temperament of the body . . . given us by nature.”
ATTACHMENT AND INDIVIDUATION
Jack P. Shonkoff
Unlike almost all other species, humans experience a prolonged period of helplessness and total dependence early in life. Consequently, the relationship between an infant and his or her primary caregiver(s) is a fundamental requirement for healthy human development.
The initial bond and growing attachment that characterize an infant-caregiver relationship are grounded firmly in biology. As described by John Bowlby, newborns and their parents are genetically programmed to form strong attachments to each other. Young infants respond preferentially to the image of a human face and to the higher pitched sound of a mother’s voice. In turn, caregivers are naturally captivated by the magnetism of a baby’s smile and the urgency of his or her cry. These core attachment behaviors have been documented in a variety of family configurations and across a broad range of cultures.6
The defining characteristics of healthy, growth-promoting, early human relationships are embodied in the social concepts of reciprocity and contingency. Thus, when young children and their caregivers are “tuned in” to each other, their interactions are adaptive. During the early years of an infant’s life, much of the responsibility for promoting a harmonious relationship rests on the caregiver’s ability to read the baby’s cues and to respond appropriately. When a caregiver’s responses are contingent, predictable, and attuned to the infant’s feelings, the young child experiences an early sense of security, personal efficacy, and positive self-worth. This leads to what Erik Erikson labeled “basic trust,” or the phenomenon through which outer predictability leads to a sense of inner certainty.7 For most parents, getting to know their babies and learning to read their signals is a highly rewarding experience that evolves naturally without the need for professional assistance. However, challenges to this relationship-building process may originate in either partner—from the infant who is relatively unresponsive or “difficult to read” as a result of prematurity, neurologic impairment, chronic illness, or extreme temperamental style; or from the caregiver whose capacity to nurture is compromised by inexperience, psychological disturbance, or severe external stressors such as poverty or social isolation.
Establishment of a secure attachment with a small number of key caregivers provides a firm foundation for healthy cognitive, social, and emotional development. Essential to this process is the need for a secure and trusted base from which the developing child can venture forth to explore the larger environment and to develop a differentiated identity as an autonomous yet socially connected individual. As the process of separation and individuation unfolds, the adaptive child navigates a delicate balance between the maintenance of strong interpersonal bonds and the mastery of both physical and psychological independence.
During the first 6 months of life, most infants respond positively to anyone; during the second 6 months, they seek preferential closeness with their primary caregivers and begin to show signs of stranger anxiety when confronted by unfamiliar persons. Throughout the second year, children and caregivers negotiate a gradual disengagement from their intense, highly personalized attachment relationship. Whereas the younger infant assumes that “mother is always there,” the emerging toddler becomes acutely aware of his or her own separateness and demonstrates varying degrees of “separation anxiety” behaviors that mark this important phase of development. For many children, a transitional object, such as a special blanket or stuffed animal, serves a vital symbolic function to facilitate the mastery of this fundamental separation challenge. By the end of the third year, most children are able to tolerate the temporary absence of their primary caregivers and accept the company of unfamiliar adults with minimal difficulty.8
Extensive research has demonstrated the far-reaching benefits of strong, early attachments and the adaptive resolution of necessary and inevitable separations. During infancy, children with secure attachments engage in richer exploratory behavior, demonstrate more sophisticated problem-solving skills, and exhibit more positive affect. During the preschool years, secure attachments are associated with better peer relationships, higher self-esteem, and a greater capacity for empathy. The ability to form increasingly mature and stable relationships into adulthood is presumed to be influenced by one’s early attachment experiences. Ongoing tensions between the development of personal autonomy and the nurturance of meaningful social relationships represent a fundamental life challenge.
MASTERY AND ACHIEVEMENT
Jack P. Shonkoff
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